The 1997–98 South-West Indian Ocean cyclone season was an event in the annual cycle of tropical cyclone formation. It ran from November 15, 1997 to April 30, 1998, except for Mauritius and the Seychelles, where it ran until May 15. These dates conventionally delimit the period of each year when most tropical cyclones form in the basin, which is west of 90°E and south of the Equator. Tropical cyclones in this basin are monitored by the Regional Specialised Meteorological Centre in Réunion.

The first advisory by the Joint Typhoon Warning Center was released on January 18, while the cyclone was inland over Mozambique.[1] Some ships reported gale-force winds from the center and was forecast to intensify. By 1200 UTC January 18, winds had decreased to 35 mph (55 km/h) and became a tropical low. The next day, the low moved southward and started to strengthen again to a 40 mph (60 km/h) minimal tropical storm. The low turned eastsouthward and scraped the Mozambique coastline and re-entered the channel. Even though the low was over open waters, the system did not strengthen and the JTWC discontinued warnings on January 23. Torrential rains poured in Mozambique and Malawi, which destroyed crops and flooded villages. This system was classified as Tropical Cyclone 13S by the JTWC.

Anacelle began its life when the Joint Typhoon Warning Center issued its first advisory on a cyclone 275 nautical miles (509 km) northeast of St. Brandon Island. The storm intensified as it moved westward and turned south on February 9. At February 10, passed very close to St. Brandon Island with maximum sustained winds of 105 mph (165 km/h). Continuing southward, Anacelle boosted itself up to 135 mph (205 km/h), its peak intensity, on February 11. Anacelle passed over Mauritius and turned southwestward. Anacelle dropped below hurricane-force winds on February 13 and became extratropical twenty-four hours later. No damage from Anacelle is available.

A tropical disturbance drifted near the coast of Mozambique to the southern tip of Madagascar in early February.[2] Early on February 9, scatterometer data indicated that a 40 mph (64 km/h) area of winds within the center. The Joint Typhoon Warning Center issued its first warnings on that days as Tropical Cyclone 21S. The system was located 225 nautical miles (417 km) south of Tulear. Tropical Cyclone 21S had formed in a high wind shear environment and was unable to strengthen further. The weak cyclone moved off to the south-southeast and weakened into a tropical depression the next day.[2]

The activity in the Mozambique Channel continued with a tropical disturbance forming on February 16. Convection began to organize very quickly, turning the disturbance into a tropical cyclone named Beltrane.[3] By 600 UTC, Beltrane moved 150 nautical miles (280 km) northwest of Tulear, Madagascar, with winds of 40 mph (64 km/h). Beltrane peaked at 45 mph (72 km/h) winds on February 16 at 1800 UTC off the southern tip of Madagascar. By the morning of February 18, all convection in Beltrane was sheared apart. The last Joint Typhoon Warning Center advisory was released that evening, in the Mozambique Channel, 100 nautical miles (190 km) from its point of origin.[3]

On February 16, the well organized remnant tropical depression of Cyclone Katrina-Victor had crossed 90°E and moved into the South-West Indian Ocean. During that day the Mauritius Meteorological Service named the system Cindy before it degenerated into a tropical disturbance later that day as the systems low level circulation, became unorganized. The disturbance then dissipated during February 19 after it had recurved towards a polar trough located near the edge of the tropics.[4]

The first advisory on Tropical Cyclone 24S was issued on February 16 by the Joint Typhoon Warning Center, about 500 nautical miles (930 km) east of La Reunion.[5] At that time, Tropical Cyclone 24S had winds of 40 mph (64 km/h). Tropical Cyclone 24S moved to the southeast on February 17, but moved eastward the next day. Wind shear and low temperatures made it impossible for 18th. The final warning on Tropical Cyclone 24S was released on February 19 after being forecast to merge with cold front in the forecast period.[5]

The Joint Typhoon Warning Center on Guam issued their first bulletins on a developing depression in the west-central South Indian Ocean on March 6.[6] The depression became a storm on March 7 with winds of 45 mph (72 km/h). Donaline began by drifting generally to the east, but by March 8, Donaline had started moving south. At this time, the storm had reached its peak intensity of 65 mph (100 km/h). Afterwards, Donaline began to weaken and accelerate to the south-southeast. Donaline's low-level circulation had becoming completely exposed to shear on March 9 and the final warning was released on March 10, when it was downgraded to a depression.[6]

The Joint Typhoon Warning Center issued their first bulletin on a rapidly developing tropical system on March 12. Elsie moved in a south-southwesterly direction for the next five days and reached its peak of 105 mph (165 km/h) on March 13. The cyclone at this point was only 1,000 nautical miles (1,900 km) east of Rodrigues Island. Elsie had weakened as fast as it had intensified and by March 14, the storm was encountering wind shear. The south-southwestern motion stopped on March 17 and the final warning was issue twelve hours later. The final warning indicated that Elsie was finish its extratropical transition within 24 hours.

The Joint Typhoon Warning Center issued their first bulletin on Tropical Cyclone 28S on March 17.[7] The initial warning indicated winds of 45 mph (70 km/h) and conditions were right for more strengthening. A passing shortwave inhibited future development and decreased the winds to 40 mph (64 km/h). During this time Fiona had drifted westward to an area about 215 nautical miles (398 km) northeast of Mauritius. The weak Fiona moved to the southeast and curved back to the west later. After the trough lifted, Fiona gained some intensity but wind shear prohibited further development. The storm passed within 75 nautical miles (139 km) east of Mauritius at around 1500 UTC on March 19. Continuing south, Fiona weakened and the final advisory was issued on March 20.[7] Winds of 44 mph (71 km/h) were reported in the eastern section of Mauritius, but no rainfall or pressure measurements were recorded.[7]

The Joint Typhoon Warning Center issued their first bulletin about a developing tropical depression on April 7.[8] The depression had maximum sustained winds of 35 mph (56 km/h). However, at the same time as Tropical Depression 32S foming, the JTWC issued a bulletin on the newly formed Tropical Depression 33S, which was located to the east-southeast. By 1200 UTC on March 7, the two systems had merged, resulting in a broad and weak wind field. Following the 32S-33S merger, the depression strengthened and was given the name Gemma on April 9. Gemma reached 65 mph (105 km/h) winds and halted its westward motion, turning it into a south-southeasterly motion. On April 11, Gemma reached its peak intensity of 80 mph (130 km/h). Afterwards, Gemma began to weaken due to wind shear separating the system.[8]

Microwave imagery on April 12 indicated that low-level features were still intact, despite the wind shear.[8] By April 13, the weakening Gemma was down to 50 mph (80 km/h) winds and the next day, Gemma was falling apart. The Joint Typhoon Warning Center issued their final bulletin about Gemma on April 15, just 230 nautical miles (430 km) east of its point of origin.[8]

The Joint Typhoon Warning Center issued their first bulletin on the newly formed Tropical Depression 33S on April 7 at 0000 UTC. The depression was only 300 nautical miles (560 km) southeast of Tropical Depression 32S (which later became Tropical Cyclone Gemma). Tropical Depression 33S moved northeastward while Tropical Depression 32S moved eastward. By 1200 UTC April 7, the center of TD-33S had pulled in closer to TD-32S. The depressions merged the same day, ending the life of Tropical Depression 33S. Maximum sustained winds from Tropical Depression 33S were 35 mph (56 km/h).[9]

The Joint Typhoon Warning Center began tracking the newly-formed Tropical Cyclone 34S on April 19 near Diego Garcia.[10] The cyclone was expected to reach hurricane-status, but it did not reach above gale intensity due to wind shear. Maximum sustained winds of 45 mph (72 km/h) were recorded on April 19 and April 20. A scatterometer pass on the next day found that the system was weakening and lessening in its circulation. The Joint Typhoon Warning Center quit tracking the storm on April 22, as the cyclone weakened into a depression.

The South Indian Ocean basin, the tropical cyclone season is considered to end on July 31 while the Joint Typhoon Warning Center (JTWC) considers it to begin on July 1. Therefore, RSMC La Reunion regarded this system as the last of the 1997-98 season, while the JTWC regarded it as the first of the 1998-99 season, and classified it as Tropical Cyclone 01S.

La Reunion issued the first advisory for this disturbance on July 20 while it was centered about 800 nautical miles (1,500 km) east of Diego Garcia. For the next couple of days the system moved slowly toward the west-southwest. On July 22, it was upgraded to a tropical depression by La Reunion. Tropical Depression H4 was already weakening on the 23rd when the JTWC issued its first advisory, placing the center about 300 nmi (560 km) southeast of Diego Garcia. The system experienced considerable vertical wind shear throughout its life, but by July 23, the hostile conditions had abated somewhat. Given its proximity to Diego Garcia, the JTWC decided to initiate advisories on the system.[11] In post-operational analysis, the JTWC upgraded the system to a tropical cyclone on the 22nd.[12]

Storm names

A tropical disturbance is named when it reaches moderate tropical storm strength. If a tropical disturbance reaches moderate tropical storm status west of 55°E, then the Sub-regional Tropical Cyclone Advisory Centre in Madagascar assigns the appropriate name to the storm. If a tropical disturbance reaches moderate tropical storm status between 55°E and 90°E, then the Sub-regional Tropical Cyclone Advisory Centre in Mauritius assigns the appropriate name to the storm. A new annual list is used every year so no names are retired.[13]